Element for initiating propellant

ABSTRACT

Apparatus for breaking rock which includes a cartridge ( 16 ) which forms an enclosure, a propellant ( 24 ) inside the enclosure, and at least one element ( 26 ) which is electrically energisable and which is made from inert material exposed to the propellant.

BACKGROUND OF THE INVENTION

This invention is concerned generally with a customized low energymethod of breaking rock in a controlled manner.

As used herein the word “rock” includes rock, ore, coal, concrete andany similar hard mass, whether above or underground, which is difficultto break or fracture. It is to be understood that “rock” is to beinterpreted broadly.

A number of techniques have been developed for the breaking of rockusing non-explosive means. These include a carbon dioxide gaspressurisation method (referred to as the Cardox method), the use of gasinjectors (the Sunburst technique), hydrofracturing and various methodsby which cartridges containing energetic substances pressurise the wallsor base of a sealed drill hole to produce penetrating cone fractures(known as PCF).

These techniques may be an order of magnitude more efficient thanconventional blasting in that they require approximately 1/10 of theenergy to break a given amount of rock compared to conventional blastingusing explosives. The lower energy reduces the resulting quantity of flyrock and air blast and to an extent allows the rockbreaking operation toproceed on a continuous basis as opposed to the batch-type situation,which prevails with conventional blasting.

Most non-explosive rockbreaking techniques rely on the generation ofhigh gas pressures to initiate a tensile fracture at the bottom of arelatively short drill hole.

As used herein the term “propellant” is to be interpreted broadly toinclude a blasting agent, propellant, gas-evolving substance, explosiveor similar means which, once initiated, generates high pressure jetmaterial typically at least partly in gaseous form. Propellants of thisnature are known in the art. “Blasting agent” and “propellant” are usedinterchangeably in this specification.

SUMMARY OF INVENTION

The invention provides apparatus for breaking rock which includes acartridge which forms an enclosure, a propellant inside the enclosure,and at least one element which is electrically energisable and which ismade from inert material exposed to the propellant.

As used herein “inert material” means a material which, unlessenergised, cannot give rise to a spark or other phenomenon which caninitiate the blasting agent.

The element may be in the nature of a filament or electrical resistor.

The element may be made from any appropriate material but a preferredmaterial is carbon, eg. in the form of graphite.

The element may be treated in any appropriate way to enhance theefficiency with which the propellant is initiated. For example theelement may be dipped in or coated with aluminium powder or any similaragent which gives rise to a sputtering effect when the element isenergised.

The element may be in the nature of a fusible link ie. a portion of theelement may be disintegrate when an electric current in excess of apredetermined amount is passed through the element. Disintegration ofthe element gives rise to the generation of high energy and relativelysmall particles which are propelled into the propellant thereby toinitiate combustion of the blasting agent. This approach should becontrasted with a technique wherein the element in the form of afilament wire is heated by the passage of electric current to a highertemperature at which the propellant is initiated but wherein thefilament remains integral and does not disintegrate due to the directeffect of the electric current passing through the filament.

The filament may be coated to prevent the propellant, or moisture in thepropellant, from reacting or fusing with the material contained in thefilament.

The filament may for example be a wire or band made from aluminium,nickel-chrome, carbon or a similar material, or a resistor such as aceramic metal device which, when energized, does not fuse ordisintegrate but instead, remains integral despite dissipatingsufficient heat to initiate combustion of the propellant.

The cartridge may include a base and a side wall which extends from thebase. The side wall may be generally cylindrical.

The cartridge may be made from a malleable material which, in thissense, includes a material which is capable of plastic deformation,without fracture, at least to a predetermined extent.

The cartridge is preferably made from a plastics material eg.polypropylene, polyethylene or the like. The scope of the invention isnot limited in this regard.

An upper end of the cartridge, ie. an end which opposes the base, may bedomed and the filament may be located at the upper end.

It falls however within the scope of the invention to locate thefilament at an intermediate location ie. between opposing ends of thecartridge.

The filament may be relatively small and thus, when energized, cause theproduction of a localized relatively high energy hot spot. On the otherhand the filament may be elongate so that, when energized, thepropellant is initiated over a fairly substantial length or area or at aplurality of points.

It falls within the scope of the invention to include more than oneelement in the cartridge so that initiation of the propellant takesplace at more than one location. Electrical leads to the element may bepositioned inside the cartridge or on an outer surface of the cartridgebut preferably are embedded in the material from which the cartridge ismade.

The electrical leads may be connected to terminals to facilitateconnection of the leads to a control unit. The terminals are preferablyon an outer side of the cartridge.

The terminals may be covered with a removable closure. Alternatively theterminals may be covered with a frangible or breakable closure. Theclosure is designed to protect the terminals from damage or exposureprior to use of the cartridge.

The element, or elements, as the case may be, with suitable leads orconductors to the elements and, where applicable control devices such astiming circuits and capacitors or other energy sources for operating thetiming circuits, may be mounted on a suitable substrate or motherboard,to facilitate handling of these components and assembly thereof togetherwith the remainder of the cartridge.

The invention also extends to an element of the aforementioned kind, andto a substrate which carries the element and, where required, one ormore components for use with, or required to energize, the element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference tothe accompanying drawings in which:

FIG. 1 is a view in cross section and from the side of blastingapparatus according to one form of the invention,

FIGS. 2 and 3 are views similar to FIG. 1 of blasting apparatusaccording to second and third forms of the invention,

FIG. 4 is an enlarged cross sectional view of a portion of a cartridgeillustrating connecting terminals on the cartridge, and

FIG. 5 shows a substrate which carries a filament, for use in theapparatus of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 of the accompanying drawings illustrates a hole 10 which isdrilled into a rock mass 12 from a face 14 using conventional drillingequipment. A cartridge 16 is loaded into the hole. In this example thecartridge has a base 18 and a generally cylindrical wall 20 whichextends from the base and which terminates at an upper end, remote fromthe base, in a rounded shape 22.

The cartridge is made from a plastics material using injectiontechniques which are known in the art. The cartridge is for example madefrom a high density plastics material such as high densitypolypropylene.

It is desirable to form the cartridge from a malleable material whichenables the cartridge to be plastically deformed, without rupture, atleast to a predetermined extent eg. of the order of 10%, or more.

The cartridge forms an enclosure for a propellant material 24 which isof known composition. The propellant is loaded into the cartridge underfactory conditions using techniques which are known in the art.

An initiator 26 is located at an upper end of the cartridge. Theinitiator has an element when, in this case, is in the form of afilament made from inert material such as carbon wire which, preferably,is formed into a coil or with a zig zag configuration. The filamentextends from two leads 30 and 32 which pass through the wall of thecartridge so that filament is exposed, in the interior of the cartridge,to the propellant 24. The leads 30 and 32 extend to a blasting controlunit, not shown, of a type which is known in the art.

Stemming 40 is placed into the hole from the rock face covering thecartridge to a desired extent and is consolidated by being tamped inposition.

The filament is energized by sending a signal of a determined energycontent through the wires 30 and 32 to the filament. The filament isthereby heated and glows creating, in effect, a localized hot spot whichtransfers sufficient energy to the propellant, in the immediate vicinityof the filament, to cause ignition of the propellant.

The propellant, when ignited, causes the release of high pressure jetmaterial which is substantially in gaseous form. This material producesa shock wave which is used to fracture the rock 12, typically with aninitial fracture being established at the bottom 40 of the hole 10.

The carbon filament 26 is, as noted, inert and consequently it ispossible to provide the cartridge 16 in a form which is ready for use inthe sense that the propellant 24 can be loaded into the cartridge eventhough the initiator 26 is already fixed to the cartridge. In many otherinstances it is not possible to provide the cartridge, loaded withpropellant, if the initiator is already fixed to the cartridge for it ispossible inadvertently to energise the initiator and thus cause unwantedcombustion of the propellant. Due to the fact that the filament is madefrom an inert material it is believed that this danger is effectivelyeliminated.

The filament may be coated with aluminium paint which, when heated,increases the quantity of energy which is released and, depending oncircumstances, can give rise to a sputtering effect which enhances theefficiency with which the propellant is ignited.

The filament could alternatively be in the form of a wire or band andcould be made from aluminium or nickel-chrome. These materials may be ofa kind which, when heated by electric current, fuse and disintegrate. Ina variation use is made of a suitable resistor, made for example from aceramic/metal composition which, when heated, glows but does not spatteror disintegrate, and which provides the necessary “hot spot” to initiatecombustion.

The arrangement shown in FIG. 2 is similar in many respect to what isshown in FIG. 1 and where applicable like reference numerals are used todesignate like components. The filament initiator 26 is, however, inthis case located at a position more or less midway between the base 18and the rounded upper end of the cartridge. The leads 30 and 32 extendfrom an upper entry point to the filament and are embedded in the wall20 of the cartridge. The filament 26 is fired in the same way as whathas been described in connection with FIG. 1.

In the arrangement shown in FIG. 3 the filament is not localized in theway shown in FIGS. 1 and 2 but, instead, is elongate. The filamentextends from end points of wires 30 and 32 which, as is the case withthe FIG. 2 embodiment, are embedded in the side wall 20. The filament isshaped into the form of a ring which extends around an internal surfaceof the wall 20. Consequently, when the filament is initiated, combustionof the propellant 24 takes place over an extended length or area or,otherwise put, at a plurality of points.

The filament 26 may be designed and operated so that when energized itis heated to glow and cause a localized temperature increase ofsufficient magnitude to initiate the blasting agent. Alternatively thefilament may be in the form of a fusable link such that a region of thefilament is heated to disintegration point by the passage of electricalcurrent. Components of the filament which are released upondisintegration are extremely hot and a sputtering-type action results asthe filament disintegrates.

It is possible to enhance the combustion effect of the filament bycoating the filament at one or more locations with an agent such asaluminium powder or any other substance which is explosive or flammableby nature. Again small localized hot spots are generated when anappropriate electrical current is passed through the filament andcombustion of the propellant is therefore initiated effectivelysimultaneously at a plurality of locations.

Also, to protect the filament against reacting with the propellant, ormoisture in the propellant, the filament may be coated with an inertmaterial such as a suitable lacquer, eg. of nitrocellulose.

FIG. 4 is an enlarged view in cross section of an upper end 22 of acartridge which may be of the type shown in any one of FIGS. 1 to 3. Theleads 30 and 32 are, as has been described, embedded in the wall 20 ofthe cartridge and terminate in relatively rigid terminals 50 and 52respectively, which project outwardly from the cartridge. The terminalsare covered by means of a cap 54 which protects the terminals duringtransport and storage of the cartridge. If the terminals are to beaccessed to enable electrical connections to be made to the terminalsthen the cap is removed. The cap may for example be threadedly engagedwith the cartridge. Alternatively the cap may be formed substantiallyintegrally with the cartridge or be secured thereto in a manner whichinhibits removal of the cap. In this case the cap includes a lid 56which is breakable, or which can be tom from the remainder along a lineof weakness, not shown, to expose the terminals so that electricalconnections can be made thereto.

FIG. 5 illustrates another modification which can be made to theaforementioned principles. FIG. 5 shows a cartridge 10 which is filledwith propellant 24. A substrate 60 is located in the cartridge. Thesubstrate is made from an inexpensive non-electrically conductivematerial, and may be in the nature of a printed circuit board, be formedfrom a suitable plastic material, or the like. The substrate may berigid or flexible.

Discrete components may be mounted directly to the substrate usingtechniques which are known in the electronics art. Alternativelycomponents may be formed on the substrate using deposition techniquessimilar to those employed in the manufacture of printed circuit boards,integrated circuits and the like.

Thus, using an appropriate technique, a filament 26 is formed on, or ismounted to, the substrate and leads 62 are formed connecting thesubstrate to a timing circuit 64. The circuit is also formed on orbonded to the substrate and includes leads 66 which extend to terminals68. The conductors 30 and 32, which are inherently more robust than theleads 62 and 64 on the substrate, are directly connected to theterminals.

The use of the substrate to carry the filament considerably facilitatesthe manufacture of the filament and its use and helps in reducinginadvertent damage to the filament which could arise during manufactureof the cartridge or when the cartridge is installed in a hole in a rockface. The substrate may be of any appropriate shape or size so that whenthe substrate is positioned inside the cartridge the filament 26 issubstantially automatically positioned at a desired location inside thecartridge. Clearly it is possible to form a plurality of filaments onthe substrate so that the individual filaments are at preciselydetermined locations relatively to each other inside the cartridge.

1-8. (canceled)
 9. Apparatus for breaking rock which includes acartridge which forms an enclosure, a propellant inside the enclosure,and at least one element which is electrically energisable, which ismade from inert material exposed to the propellant and which, whenenergised, initiates combustion of the propellant to release highpressure material inside the cartridge.
 10. Apparatus according to claim9 wherein the element is made from a material selected from carbon,graphite, aluminum, and nickel-chrome.
 11. Apparatus according to claim9 wherein the cartridge is made from a malleable material and includes acylindrical side wall, a base and a domed end which opposes the base.12. Apparatus according to claim 9 which includes electrical leads whichextend from terminals on an outer side of the cartridge to the element.13. Apparatus according to claim 12 which includes a removable closurewhich covers the terminals.
 14. Apparatus according to claim 9 whichincludes a substrate on which the element is mounted and which isembedded in the propellant.
 15. Apparatus according to claim 14 whichincludes a control device, for controlling operation of the element, onthe substrate.
 16. Apparatus according to claim 9 wherein the element isin the form of an elongate filament.